/*
 * Copyright (c) 2018. Manuel D. Rossetti, rossetti@uark.edu
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

package jsl.utilities.random.rvariable;

import jsl.utilities.random.rng.RNStreamIfc;

/**
 * Exponential(mean) random variable
 */
public final class ExponentialRV extends ParameterizedRV {

    private final double mean;

    /**
     * Defaults to mean = 1.0
     */
    public ExponentialRV() {
        this(1.0);
    }

    /**
     * Uses a new stream from the default stream factory
     *
     * @param mean must be greater than 0.0
     */
    public ExponentialRV(double mean) {
        this(mean, JSLRandom.nextRNStream());
    }

    /**
     * @param mean      must be greater than 0.0
     * @param streamNum the stream number
     */
    public ExponentialRV(double mean, int streamNum) {
        this(mean, JSLRandom.rnStream(streamNum));
    }

    /**
     * @param mean must be greater than 0.0
     * @param rng  must be null
     */
    public ExponentialRV(double mean, RNStreamIfc rng) {
        super(rng);
        if (mean <= 0.0) {
            throw new IllegalArgumentException("Exponential mean must be > 0.0");
        }
        this.mean = mean;
    }

    /**
     * @param rng the RngIfc to use
     * @return a new instance with same parameter value
     */
    public ExponentialRV newInstance(RNStreamIfc rng) {
        return new ExponentialRV(this.mean, rng);
    }

    @Override
    public String toString() {
        return "ExponentialRV{" +
                "mean=" + mean +
                '}';
    }

    /**
     * @return the mean value
     */
    public double getMean() {
        return mean;
    }

    @Override
    protected double generate() {
        return JSLRandom.rExponential(mean, myRNStream);
    }

    /**
     * The parameter name is "mean"
     *
     * @return parameter for Exponential random variables
     */
    @Override
    public RVParameters getParameters() {
        RVParameters parameters = new RVParameters.ExponentialRVParameters();
        parameters.changeDoubleParameter("mean", mean);
        return parameters;
    }
}
